To add to what Dirtdevil has said, hotboxes have a wide variety of uses, I use one on all the above uses listed as well as heating the tires before mounting them on the wheels, I've put my Kart wraps on the lid while heating my tires for mounting, to make them more pliable. IMO, hotboxes aren't really a necessity, but they'll make karting a lot easier.The number one use for a hotbox is heating tires to prep , and to keep your tires at even warm temp on those race days. Some folks use them to grow tires for stagger, if you heat a tire the air inside expands also makes the rubber softer. Rolling tires in the box you place your tires on a set of rollers which are very slowly turned by an electric rotisserie motor (similar to most high end barbeque grills). You typically place prep in a prep tray directly under the tires to prep them from the outside, or place prep inside of the wheel and tire assembly through the valve stem via a turkey baster or similar method, to "rotisserie" prep them from the inside out.
There are two main kinds of hotboxes out there, some you need to purchase a heat gun basically blow the heat in the box, this box is usually less expensive 300 - 500. The preferred box by most is a heat strip box, this box has a digital thermostat and you simply set your temp plug it in. Also has a prep pan in the bottom that is removable. However, these boxes are usually 700 +. You can check out the one we make by going to my facebook page https://www.facebook.com/josh.spicerenterprises. Hope this helps.